Glaucoma (abnormal intra-ocular pressure) is a major eye problem that leads to blindness in a significant percentage of the world population. Glaucoma is the most common cause of blindness in the world today. The established technique for treating glaucoma is drug based. Alternative treatment modalities have been sought to avoid the side effects and non-specificity associated with drug based treatments. Over the past few years a technique known as selective laser trabeculoplasty (SLT) has been invented by Latina. The technique is described in U.S. Pat. No. 5,549,596, assigned to The General Hospital Corporation. Latina describes the use of a frequency doubled Nd:YAG laser for the SLT procedure.
SLT is an improvement over a previously used technique referred to as argon laser trabeculoplasty (ALT). ALT uses a thermal effect to coagulate loose trabecular meshwork cells believed to be present in patients with glaucoma. Because an Argon laser is essentially CW (if pulsed, the pulse duration is long compared to thermal transfer mechanisms) there is significant heat transfer into surrounding tissue. This results in damage to otherwise healthy cells. It has been found that the ALT process can only be used once or twice before collateral damage prevents any further benefit from ALT treatment.
In contrast, SLT utilizes a pulsed laser (the pulse duration is short compared to thermal effects) so there is minimal heat transfer to surrounding tissue. SLT has been found to be repeatable, unlike the ALT process.
A detailed discussion of the SLT modality and a comparison with ALT is found in Ocular Surgery News published 1 Mar. 2000.
Another very common ophthalmic treatment is secondary cataract surgery. The most effective laser for secondary cataract surgery is a Nd:YAG laser operating at 1064 nm. These lasers are typically referred to as photodisruptors as they act by non-thermal mechanisms to cut tissue. A typical ophthalmic laser system consists of the laser head and a beam delivery system coupled to a conventional slit lamp assembly. A typical laser system for secondary cataract surgery is described in U.S. Pat. No. 6,325,792.
At present, two separate laser systems are necessary to perform the procedures for treating the two most common eye problems.
An attempt to address the problem of requiring multiple lasers for different treatment modalities has been described in U.S. Pat. No. 6,066,127. This patent describes a system for changing the laser cavity between a pulsed configuration and a continuous wave configuration by introducing a movable intracavity element. This approach is problematic because it is extremely difficult to maintain optimum alignment of the laser cavity with a movable intracavity element.
A better solution is required.